The present invention relates to a process for production of diene-rubber polymer latexes. The process can notably improve yields in the production of industrially useful diene rubber polymer latexes.
Diene rubber polymers are widely known as elastomers used in ABS resins, MBS resins and the like, and they are usually produced by emulsion polymerization. Such emulsion polymerization has traditionally been carried out by batch processes. In the case of batch polymerization, however, heat removal efficiency is low with larger volume vessels. When larger-sized vessels are used, therefore, fluctuations in the polymerization rate can hamper heat removal of the polymerization heat, often leading to an uncontrolled reaction. This reduces reproducibility of the latex product between batches, and can make it impossible to achieve stable production.
In order to prevent such a reduction in reproducibility of the latex product between batches, there has already been proposed, in Japanese Examined Patent Publication No. 54-3511, an improved batch production process aimed at controlling the polymerization temperature during batch polymerization reactions. In this process, polymerization is carried out while continuously or intermittently adding the monomer and a liquid composed mainly of water to the reactor, which is kept at high activity under high temperature conditions. The process prevents uncontrolled polymerization reaction since the unreacted monomer is not allowed to reside in large amounts in the polymerization system during the reaction, while a heat removal effect is obtained by slight sensible heat corresponding to the difference in the reaction temperature induced by the dropwise addition of water, and it is therefore possible to control the polymerization temperature in a more stable manner than with ordinary batch polymerization.
At the same time there has been a growing demand for further improved productivity in recent years as diene rubber polymers find more and more useful applications. However, despite the more satisfactory control of the polymerization temperature compared to other batch reactions, only minimal improvement in heat removal efficiency can be achieved because of the small amount of water that is added dropwise. That is, while the aforementioned publication discloses that dropwise addition of the monomer allows control of the polymerization rate and that dropwise addition of the water provides a slight heat removal-effect, no method has yet been known that takes maximum advantage of this knowledge for an adequate increase-in heat-removal efficiency and for a more notable increase in productivity.
It is therefore an object of the present invention to provide a process for the production of industrially useful diene rubber polymer latexes that affords very high productivity.
As a result of diligent research aimed at overcoming the problems mentioned above, the present inventors have completed the present invention upon finding that, in an improved batch process for the production of diene rubber polymer latexes whereby a portion of the starting material used for emulsion polymerization is added dropwise either continuously or intermittently between polymerization reactions, a notable improvement in polymerization productivity can be achieved if, during the polymerization, at least xc2xd of the total amount of monomer of the starting material used is added dropwise at a prescribed temperature while at least xc2xd of the total amount of water is also added dropwise at a prescribed temperature, in order to utilize the sensible heat of the dropwise added water.
In other words, the present invention provides a process for production of diene rubber polymers wherein, during production of a diene rubber polymer by emulsion polymerization, the polymerization is initiated using a portion of the starting material and the polymerization is continued with either continuous or intermittent dropwise addition of the remainder of the starting material during the polymerization, and wherein at least xc2xd of the total amount of the water and of the monomer in the starting material used is added dropwise during the polymerization, and the temperature of the dropwise added water and monomer is kept at least 20xc2x0 C. below the polymerization temperature.
When the size of the reactor is increased to improve productivity, the jacket cooling power per unit volume of the reactor decreases with a greater reactor volume. Thus, even in polymerization rate ranges that allow production to be easily accomplished on a small scale, the jacket cooling power decreases with larger volumes, making it impossible to remove the heat generated by polymerization. Consequently, increasing the volume of the reactor necessitates reducing the polymerization rate to a speed corresponding to the heat removal efficiency: of the reactor, i.e., it has been necessary to take measures contrary to the goal of improving productivity. There has also been considered a method of increasing the heat removal efficiency by installing an auxiliary cooling apparatus such as a vent condenser, in order to achieve stable production without reducing the polymerization rate, but it has been difficult to achieve adequate improvement in productivity by such methods.
On the other hand, when sensible heat, due to dropwise addition of water, is utilized according to the process of the invention, the greater charging amount with the increased volume of the reactor requires a greater amount of dropwise water addition, and thus the heat removal efficiency increases in proportion to the scale of the apparatus. Consequently, using the process of the invention for a reactor with a large-sized volume can very efficiently improve productivity. However, the function and effect of the invention are not limited only to this type of situation, and for example, even when polymerization is conducted at a polymerization rate that allows sufficient heat removal with the heat removal efficiency of the polymerization reactor, the process of the invention may be applied for a notable reduction in the cooling load of the jacket-or condenser, for a highly effective result in terms of energy savings.
The following may be mentioned as objects for application of the improved batch production process of the invention.
(1) Improvement in heat removal efficiency by utilizing the sensible heat of dropwise addition of water and the monomer;
(2) Avoidance of situations in which the polymerization reaction temperature cannot be controlled; and
(3) Energy savings.
As concerns (1), dropwise addition of a larger amount of water compared to the prior art, of at least xc2xd of the total amount of water which has the largest sensible heat, the sensible heat corresponding to the difference between the reaction temperature and the temperature of the water that is supplied is thereby utilized for heat removal. As a result, it is possible to achieve an improved heat removal efficiency that-has not been possible by the prior art, and to thereby notably improve productivity.
As concerns (2), since the total amount of monomer is not charged from the start, as is done in the prior art, it is possible to control the reaction temperature by stopping the dropwise addition of the monomer, even when the polymerization reaction temperature can no longer be controlled.
As concerns (3), in normal batch polymerization the entire amounts of the water, monomer, emulsifier, etc. are charged prior to polymerization, and after heating to the prescribed temperature, the initiator or catalyst is introduced to start the polymerization. That is, heat energy is necessary to raise the charging temperature to the polymerization initiation temperature. According to the present invention, however, the amount of water and monomer in the initial charging composition that is charged before polymerization is no more than xc2xd compared to a batch process. Consequently, the energy required for a temperature increase can be reduced to correspond to the initial charging amount, so that the required energy can be considerably reduced compared to a batch process. In addition, a batch process has a very high polymerization rate during the stage after polymerization initiation known as the xe2x80x9czero-order reaction periodxe2x80x9d (when the conversion rate is in the range of about 0-70%), and the cooling load during this period is very high. However, according to the present invention, which utilizes the sensible heat of the water and monomer, the cooling load on the jacket and condenser can be notably reduced to provide a very high energy savings effect.
According to the invention, the improved heat removal efficiency due to the sensible heat of the water is utilized so that the charging amount may be increased or the polymerization concentration raised by the degree of improved heat removal efficiency, for greater productivity.